The gas bladder as a hydrostatic organ in Thymallus thymallus L., Osmerus eperlanus L. and Mallotus villosus Müll

1. The gas bladder of Thymallus thymallus is investigated concerning the topography, vascularization, histology and histochemistry. Simple micro-retia mirabilia are found, forming a counter-current vascular system of probably very low exchange capacity. The epithelium, lining the lumen, is probably one of very low activity. If a secretion of gases into the bladder occurs, it is probably very slow. 2. The gas bladder of Osmerus eperlanus is investigated as described above. The blood vessels are not arranged as a counter-current system, though capillaries reach the base of the epithelium. The epithelium, which is also EM investigated, contains apically considerable amounts of mucopolysaccharides and glycogen. A secretion, directly to the lumen, of gases or of a matter which can be transformed into gases, is proposed. 3. The gas bladder of Mallotus villosus is investigated as described under point 1. Besides, low power electron micrographs has been studied, and analyses of the gaseous content of the bladder were made after administration of pilocarpine, dibenzyline and yohimbine. No counter current vascular system is present but capillaries reach the base of the epithelium. The epithelial cells contain and probably secrete polysaccharides (though not glycogen) directly to the lumen. The gas is composed of 0-2% carbon dioxide, 0-18% oxygen and 80-100% inert gases. The administration of drugs did not in any case increase the percentage of oxygen or carbon dioxide, compared with the control animals. It is proposed that no gas secretion occurs. 4. The muscle layer of the gas bladder is in all the three species continuous with the muscularis externa of the oesophagus. This argues for a neutral terminology concerning the layers of the gas bladder wall. 5. A muscular sphincter is morphologically documented at the debouch of the pneumatic duct into the oesophagus. Supported by grants from the Faculty of Mathematics and Science of the University of Lund. Photos by B. Henriksson.